For optical instruments used in industrial environments, it is critical to validate the results produced by the instrument without the need to remove the instrument from the process. For instruments using a static intensity signal, for example turbidity meters, this is commonly done by periodically switching the light path by means of a movable mirror or other moving optical devices to observe a “standard” target of known optical properties. If the frequent reading from such a standard remains identical to a first measurement of the standard after calibration, it can be assumed that the measurement results obtained by the instrument are still valid.
This procedure can be carried out while the instrument is inserted into a process environment, yet it requires moving optical elements and a schedule to carry out the standard reference measurements. It would be preferable if no additional moving elements were necessary and the reference measurement could be carried out automatically.
For a scanning optical device, such as the ones based on the Focused Beam Reflectance Measurement (FBRM) as disclosed in U.S. Pat. No. 4,871,251 explicitly incorporated herein by reference but not limited to it, no suitable validation system exists. The present invention describes a method and apparatus for validating a scanning device, including a FBRM-based scanning device.